Recently, for the purpose of supplementing a driver's vision and/or providing input information for use in image processing for driving assistance, an on-board camera is often mounted on a vehicle for capturing a visual scene around the vehicle. If an image captured by the camera is to be utilized e.g. for a parking assistance device, it is needed that the coordinate system of the image captured by the camera and the coordinate system of the image used in the image processing match with high precision each other. As a matter of fact, the designed ideal correlation therebetween can hardly be obtained due to error or tolerance in the manufacturing precision of the camera per se and/or in the mounting precision of the camera. Accordingly, calibration of the camera has been done during the production step of the vehicle, so as to achieve matching between the two coordinate systems.
U.S. Pat. No. 6,813,371B (Patent Document 1) and JP2005-77107A1 (Patent Document 2) disclose a technique for effecting such calibration of the camera during the production process of the vehicle. According to the technique disclosed by these documents, the calibration of the camera is done through a worker's operation of an adjustment button, by causing a calibration index captured by the camera to agree on a display screen with an adjustment frame displayed in superposition with a captured image, under a predetermined criterion. As the adjustment frame, there are provided three kinds, in correspondence with roll, tilt and pan in the three-dimensional, orthogonal coordinate system. In operation, a worker will operate the adjustment button so as to place the calibration index within these adjustment frames one after another, whereby camera parameters are changed. Upon completion of all adjustments relative to the three axes of the coordinate system, the changed camera parameters are stored in a memory.
The above-described technique disclosed by Patent Document 1 and Patent Document 2 is advantageous in allowing camera calibration with the relatively simple construction. However, the worker needs to effect adjustments relative to the three axes corresponding to rotational directions of the camera by manually operating the adjustment button, while continuously viewing a display screen. So, the calibration takes a long time and the calibration precision tends to be unstable, depending on the skill/experience of each individual worker. So, there is the possibility of a product with insufficient calibration being forwarded inadvertently to the subsequent production process. Then, as shown in JP2008-131177A1 (Patent Document 3) and JP2008-131250A1 (Patent Document 4), there have been proposed a further technique that obtains camera parameters (projection parameters) through calculation of rotation matrices of the camera. According to the technique, a first matrix is calculated based on coordinates in a reference coordinate system of a calibration point set at a designed position in the reference coordinates. And, a second matrix is calculated based on coordinates in an image coordinate system of the calibration point specified from the captured image of the camera. Further, based on the first matrix and the second matrix, a rotation matrix (third matrix) of the camera is calculated.